In all the extant textual sources, we have in verse 13 “the foundation of a city” but in verse 14 “a foundation for a city”. In the original manuscript, the article in front of foundation is not extant in either case. In each instance, the lacuna is sufficiently long that either article, a or the, could have been the reading in 𝓞. It does seem strange that one clause would have the and the other would have a. In terms of English usage, I would expect the foundation in both cases, especially since these clauses parallel each other:
13 the Nephites began the foundation of a city
14 they also began a foundation for a city
Also notice that in both instances there is a following sentence that refers to the naming of the city (or land):
13 and they called the name of the city Moroni
14 and they called the name of the city or the land Nephihah
If 𝓞 read “the foundation for a city” in verse 14, then Oliver Cowdery, when copying from 𝓞 into 𝓟, must have accidentally replaced the the with a, perhaps under the influence of the a in the following prepositional phrase (“for a city”). For another instance where Oliver replaced the with a as he copied from 𝓞 into 𝓟, see the discussion under Alma 47:13 regarding the expression “the second leader over the whole army”.
Elsewhere in the Book of Mormon, a can modify foundation but only in a metaphorical sense rather than in reference to the physical foundation of an actual city or building:
“a sandy foundation” 2 Nephi 28:28, 3 Nephi 11:40, 3 Nephi 18:13
“a foundation for serious consequences” Alma 50:32
“a sure foundation / a foundation” Helaman 5:12
There are no other references in the Book of Mormon to the literal foundation of a city or building.
In the King James Bible, nearly every reference to the physical foundation of a city or a building (such as a house or a temple) uses a definite determiner, usually the definite article the. The only example where the determiner is the indefinite article a is in Luke 6:49: “but he that heareth and doeth not is like a man that without a foundation built an house upon the earth”. But here we have a negative context (“without a foundation”), so the indefinite a is expected. Whenever the context is positive, the King James Bible has only a definite determiner for the physical foundation of cities and buildings. Thus evidence elsewhere in the scriptures supports the occurrence of the with foundation, providing the reference is to cities and buildings.
Despite these arguments for emending “a foundation for a city” to “the foundation for a city” in Alma 50:14, the reading with a does work. Note that the preposition for may make a difference. The phrase “a foundation of a city” does sound strange, but “a foundation for a city” does not. Although in the transcript of 𝓞 for this part of the text I conjectured that verse 14 read “the foundation for a city”, the critical text will maintain the reading of the earliest extant source (namely, the reading in 𝓟: “a foundation for a city”).
Summary: Maintain in Alma 50:14 the reading of the earliest extant source, the printer’s manuscript: “a foundation for a city”; although the indefinite article a may be an error for the (note the phrase “the foundation of a city” in verse 13), the a works in verse 14 and will therefore be retained.